A standard GPS receiver determines its position relative to the satellite constellation by measuring propagation time of signals transmitted simultaneously from Navstar satellites. This constellation consists of 24 satellites each orbiting the earth in 12 hours time, such that any user has always a line of sight to 5-10 satellites. Each satellite transmits a continuous pseudorandom (PRN) noise sequence with a chip rate of 1023 KHz, and a repetition period of 1023 chips. Each satellite has its own particular PRN sequence, which has good correlation properties and is orthogonal to the PN sequences of the other satellites.
Superimposed on the PRN sequence is a satellite data message. The data message includes positioning data (Almanac and Ephemeris), clock timing data and time of week (TOW) data. The GPS processor locks on a specific visible satellite signal using correlation techniques often performed by a correlator, to correlate between the received signals and a known replica of the transmitted signal from the satellite. In urban environment and under dense canopy of trees, the satellite signals are likely to become weak, consequently decreasing the feasibility of achieving good correlation between stored replicas and a detected signal. The probability of achieving correlation and subsequent lock with such weak satellite signals increases as the accuracy of the local timing reference gets better.
After lock has been achieved, positioning and time data (ephemeris and TOW) are demodulated from the carrier signal. The correlation and data extraction sequence described above is repeated for at least three more satellite codes in order to obtain earth position of receiver in 3 dimensions. The frequency of the received GPS signal is generally shifted with respect to the satellite-outgoing signal, such that unless corrected for, a correlation with the satellite PRN sequence may not occur. The instantaneous frequency shift of the received GPS signal is composed of three instantaneous components, as described in equation 1:ω=ωs+ωm+ωclock
Where ω=frequency shift,                ωs=Satellite Doppler frequency shift,        ωm=Mobile unit Doppler frequency shift, ˜zero for a pedestrian or a known value for a known vehicle velocity,        ωclock=Mobile Unit clock shift.        
The local clock shift is the major contributing factor to the received GPS signal frequency shift, with an error reaching as high as a few ppms of the satellite signal, which in terms of frequency deviation is in the range of a few KHz of satellite signal. It is therefore of highest priority to correct its deviation. The other two contributions to frequency shift may be estimated based on a prior knowledge of the velocity parameters of the MU and of the GPS satellites. Most GPS receivers tune their local reference using a frequency search. In such a standard processing technique, the frequency shift can be estimated and a separate correlation for each estimate is performed. However, by performing the correlation for every estimate, the processing time is lengthened, resulting in excessively long time spent on obtaining a reliable correlation. In light of the above, the need for calibrating local clocks or oscillators is therefore evident. One such method is disclosed in U.S. Pat. No. 5,841,396 a method for calibrating the frequency of the local clock of a local GPS receiver by locking on to a wireless precision carrier. This carrier has a known frequency, which is used to correct the frequency of the local clock.